Long-range attraction between colloidal spheres at the air-water interface: The consequence of an irregular meniscus

Recent observations of charged colloidal particles trapped at the air-water interface revealed long-range interparticle attractive forces, not account ed for by the standard theories of colloidal interactions. We propose a mec hanism for attraction which is based on nonuniform wetting causing an irreg ular shape of the particle meniscus. The excess water surface area created by these distortions can be minimized when two adjacent particles assume an optimum relative orientation and distance. Typically, for spheres with dia meter of 1 mum at an interparticle distance of 2 mum, deviations from the i deal contact line by as little as 50 nm result in an interaction energy of the order of 10(4)kT. Roughness-induced capillarity explains the experiment al findings, including the cluster dissolution caused by addition of deterg ent to the subphase and the formation of linear aggregates. This kind of in teraction should also be of importance in particle-stabilized foams and emu lsions.